In the prior arts for turning a log by means of a veneer lathe, the turning center on each end of the log is held by a spindle chuck which can move freely back and forth to both wood end faces of the log, and then, the planer table is fed in accordance with the rotation of the spindle for producing an elementary sheet of plywood (which will be designated as an elementary sheet hereinafter). The step for centering a log should be done prior to the turning step for making the elementary sheet. The log for which the turning center has already been determined is supplied to the spindle position in the veneer lathe.
With respect to this sort of process for centering logs, the present applicant filed a Japanese Patent Application No. 295804/85 on Dec. 26, 1985, entitled "A Method and Apparatus for Centering Logs", which is incorporated here in by reference. The makeup of the invention described above comprises a pair of spindles which are arranged on the left and right sides movably in the horizontal direction. Holding claws are mounted on the tips of the spindles, and rotation angle detectors are installed in the neighborhood of the basal ends of the respective spindles.
On the other hand, a travelling body is supported which is able to travel under the guidance of the horizontal beam on the upper portion of the machine frame. Transportation claws are mounted on the travelling body enabling reciprocating and up/down motions by means of a device for x-axis adjustment or y-axis adjustment, respectively, which is suspended from each side of the travelling body. Furthermore, a plurality of rocking arms are provided while spaced arbitrarily in the longitudinal direction of the log. Each rocking arm is mounted to be able to rock, and a detector for detecting the amount of displacement is attached to each basal end thereof. When the log is rotated around a virtual center with a rough accuracy by a pair of holding claws described above, the coordinates of the turning center are calculated by both the data from the rotation angle detectors and the data from the detectors for detecting the amount of displacement. According to the resulting coordinates, the amount of adjustment in the lateral direction and that in the downward direction are sent to the device of x-axis adjustment and that of y-axis adjustment, respectively.
In this process for centering a log, first of all, the turning center has been determined from the data of cross-sectional peripheries, having centers on the virtual center axis and being perpendicular thereto, with respect to 3 cross sections (i.e., those adjacent to both the ends of the log and in the middle of the log) or 5 cross sections including additional 2 ones besides the said 3 ones in case of a longer log.
Secondly, both the amounts of X-directional and Y-directional adjustment upon the position of turning center which are obtained by using the log-centering apparatus are transmitted to the transportation claws which are placed between the log-centering apparatus and the spindle position in the veneer lathe. While the log held by these transportation claws is carried to the spindle position, each adjustment is completed according to said amount of adjustment. At the same time, the position of the planer table of the veneer lathe on the feeding shaft is controlled with reference to the maximum radius with respect to the turning center of the log determined from each of the data. Thus the impact between the blade of the planer table and the log can be avoided, and the planer table can stand by at a position ready to shave the log immediately, when the log is attached to the spindle chuck.
As is basically disclosed in Japanese Patent Publication No. 54003-1983, the method for controlling the position of the planer table is instituted so that the largest diameter of the log and the position of the planer table are both detected and the amount of movement of the planer table is controlled according to the data obtained. Thus the log is set to the spindle chuck while the planer table is kept waiting at a predetermined position.
The largest radius of the log, which has already been set to the spindle chuck, is determined from 3 cross sections arranged in the longitudinal direction of the log (i.e., those adjacent to both the ends of the log, or 5 cross sections further including two ones besides the said three ones in case of a longer log). Under the presence of the cut marks of branches or convex portions such as galls between the cross sections, the maximum radius obtained from the data is, however, to be underestimated. Therefore, the maximum permissible amount of the convex portions, which exists in the interval of each cross section calculated from data, is preliminarily taken into account and added to the maximum radius obtained by the method described above for determining the stand by position of the planing table on the feeding axis. Thereby a collision between the log and the planer table can be avoided when the log is set to the spindle chuck, and the log is ready to be shaved immediately.
Thirdly, both X-directional and Y-directional turning center adjusting amounts obtained by using the log-centering apparatus are transmitted to the transportation claws located between the log-centering apparatus and the spindle position in the veneer lathe. While the log held by the transportation claws is carried to the spindle position, respective adjustments with said amount obtained are completed, thereby the time needed for centering and setting the log can be saved.
In the first technique described above, however, logs to be processed usually have somehow bending and twisting in the longitudinal direction of the log, cut marks of branches, convex portions of galls, and further, concave portions, such as knot holes and marks due to the removal of cans. If these things exist within each space with respect to the data needed for centering the log, the accuracy of centering decreases significantly.
In the second technique, several effects are expected unless the convex portion existing in each space between the cross section in the log exceeds the maximum permissible amount in the makeup described above. In a case where the convex portion exceeds the maximum permissible amount, such a concave portion may collide with the planer table when the log is set to the spindle chuck, resulting in a breakage of the planer table. In the opposite case (i.e., no concave portion existing within each space of the cross sections of the log) the part of the maximum permissible amount newly added will become an extra space between the log set to the spindle chuck and the blade of the planer table. Therefore, the planer table is to be fed idly until the planer table reaches the position where the turning of the log is started. During this period no elementary wood is shaved out so that the production efficiency may decrease.
In the third technique, a certain degree of effect can be expected only when the horizontal space between the device for centering a log and the spindle of the veneer lathe is so open that no attachment on either device intervenes therein in the constitution described above. In cases where attachments intervene in this space, said attachments would become obstacles against the log as well as the transportation claws using on the horizontal beam in the upper portion of the machine frame as a guide. For example, in some cases, when pushing the blade of the planer table against the log for turning, a backup roller unit for receiving the pressure from the blade is mounted on the opposite side of the blade with respect to the log, and this prevents the log from deforming due to the pressure from the blade. This backup roller unit is usually placed between the device for centering a log and the spindle of the veneer lathe such that this becomes a hindrance for transporting the log.